This invention relates to data communication systems. In particular, it involves an interface which permits a wide variety of peripheral devices to be used with central control units using serial biphase data transmissions.
In general, interface systems are used to make peripheral devices such as printers, video displays, terminals, etc. compatible with a given central control unit. Often the control unit is made by one manufacturer and the peripheral device is made by another. If the operating characteristics of the control unit are different from the peripheral device, then it is impossible to use the two together. Many peripheral devices use parallel transistor-transistor logic (TTL) in which each bit of data word is operated simultaneously. In contrast, it is advantageous to transmit data from the control unit in a serial manner. Either the peripheral device or an interface must be used to convert the serial data into parallel TTL format in order for the two devices to be compatible.
In most conventional data communication systems, each bit of the serial data transmission from the control unit is represented by a constant voltage level. In other words, a voltage level of so many volts for a given bit time defines a digital zero whereas a different voltage level for that bit time defines a digital one. Since the voltage level remains constant for each bit time it is relatively easy to design an interface for converting the serial data transmissions into TTL parallel data for use by the peripheral units.
International Business Machines Corporation (IBM) has introduced its Series 3274/3276 (also known as "Type A") control units which use a dramatically different serial transmission architecture. According to the new IBM data transmission specifications, there is a voltage transition in the middle of each serial data bit. This will be referred to as a mid-bit voltage transition, or simply a mid-bit transition. A digital zero is represented by a high to low transition within a given bit time period whereas a digital one is defined by a low to high transition occuring at the mid-bit time. Each transition is accompanied by a predistortion pulse to aid in the detection of the data at remote locations. For purposes of this invention, this type of data transmission will be referred to as biphase data transmissions. FIG. 1(A) illustrates a typical biphase transmission representing two biphase zeroes followed by a biphase one.
IBM has established a new data transmission protocol which is used in conjunction with the biphase method of defining each bit. Each transmission of valid data is preceded by a header consisting of a line quiesce and a code violation. The purpose of this header is to condition the coaxial cable which carries the transmission to insure that bit and byte synchronization may be achieved. Each byte of data consists of 12 bits having a sync bit at its beginning and a parity bit at its end. The last byte of transmitted data is followed by an ending sequence as defined by IBM. FIG. 2 illustrates this protocol.
More will be said about the characteristics of such biphase data transmissions and related protocol later in the specification. However, it can be seen that conventional interface designs cannot be used to translate biphase serial transmissions to the TTL parallel format used by most peripheral units. Where in the past each bit was defined by constant voltage level for the given bit time now each bit has two discrete voltage levels for each bit time and the definition of a digital one or zero depends upon the direction of the transition. Moreover, the biphase transmissions occur at an extremely fast rate of over 2.3 megabits/second (approximately 424 nanoseconds per bit). Additionally, the voltage levels used in these biphase transmissions are relatively low. The conversion of this high frequency data transmission at low voltage levels makes it exceedingly difficult to detect and convert these transmissions into TTL parallel compatible format.
Furthermore, the task of discriminating between meaningful mid-bit transitions and meaningless between bit transitions becomes very difficult, as does the problem of sorting out valid data from invalid data used in the transmission protocol.
Additionally, the peripheral device is required to respond to commands from the control unit very quickly. This places additional burden on the interface in that it must reconstruct the header protocol, fetch and translate the TTL parallel data into biphase serial format in a timely manner. Otherwise, the control unit may assume that the peripheral device is malfunctioning and will cease communicating with it.